In injection molding of plastics, plastic is typically melted in an extruder by a rotating screw and then forced into a mold cavity. The material is allowed to cool in the mold cavity and then is ejected after solidifying. During the filling phase, the molecular orientation of the plastic can become aligned with the direction of flow due to viscous drag. This molecular orientation can have a pronounced effect on the molded part's mechanical properties. The anisotropy due to the molecular orientation is especially pronounced in fiber filled plastics. Flow of the material within the mold may also introduce another type of material defect called a weld line.
A weld line is the term used to describe the aesthetic and structural defect that occurs when a molten plastic flow front is divided and rejoined within the mold during the mold filling process. The visual defect associated with the weld line can be accompanied by an inconsistency of the material in the weld area. As a result, the strength of the molded part in the region of the weld line can be greatly reduced in comparison to surrounding areas.
Weld lines are commonly caused by core pins within the mold cavity of a mold. Core pins are usually employed to form holes within molded parts. FIG. 1, depicts a prior art mold 10 having a core pin 114. The core pin 114 is located within mold cavity 112 of mold 110 to form a hole in the molded part. When plastic is injected into mold cavity 112 via runner 122 and gate 116, the core pin 114 divides the flow of plastic within cavity 112. The divided flows of plastic flowing around core 114 partially solidify by the time they rejoin downstream of core 114. As a result, the two flows of plastic are unable to fully mix with each other when rejoined and a weld line 120 forms downstream of core 114. The weld line may have a small V-notch on the surface of the molded part where the two plastic flow fronts meet due in part to volatiles and gases which are trapped in the weld line area. The V-notch may be prominent enough to cause aesthetic problems as well as form a stress concentrating region. Weld lines are also commonly caused by complex flow patterns in a mold due to variable wall thicknesses or multiple flow entrances to a mold.
Several techniques have been used to improve the weld line strengths of injection molded parts. One such method is disclosed in U.S. Pat. No. 3,213,492 in which an injection mold and its core are moved laterally relative to each other to knead material within the mold. This causes the material that has flowed around the core to homogeneously fuse and bond in order to strengthen the weld line caused by the core. A problem with this approach is that its usefulness is limited by the geometry of the molded part since the molded part must have a hole. Additionally, a molded part such as large flat rectangular plate with a central hole which is filled from one end will have little benefit from this technique. This is due to the fact there will be little kneading effect on the material outside of the range of the kneading motion. As a result, it is possible for a large portion of a weld line to be unaffected by the kneading motion. Another problem of this approach is that the apparatus is relatively complex and expensive. Furthermore, moving the mold and core relative to each other while a part is solidifying may cause damage to a an intricate or delicate mold.
Another present method is disclosed in U.S. Pat. Nos. 5,059,368, 5,156,858 and 5,160,466 in which an oscillating piston or pistons external to the mold cavity oscillate the molding material back and forth within the mold to mix the material and eliminate or strengthen weld lines. This equipment is external to the mold and communicates to the material in the mold through several conduits. The placement of the conduits relative to the mold controls the location within the mold in which mixing occurs. A problem with this approach is that the apparatus is external to the mold and is rather large, complex and costly. The part geometry must also be suitable for this method because orifices which allow the oscillating plastic flow to enter and exit the mold cavity must be placed so as to be opposite each other with the desired affected area laying in between these orifices.